f# one list to another?

I have a list of tuples with three values in tuples

I want to create new List of strings out of previous list with one value out of tuples.

List [(string * st开发者_开发百科ring * int) ]

List[ for i in columns -> i.getfirstvalueintuple]

How can i do that? very basic question but i can't figure it out.

Also is there any other way of building another kind of list or seq out of existing list?

seq { for (first, _, _) in lst do yield first };;

This gives a sequence. You can also use List.map:

lst |> List.map (fun (first, _, _) -> first);;

Which gives a list.

You just need a List.map with a 'project' function:

let newList = List.map (fun (x, _, _) -> x) orgList

The List.map takes a function and a list and applies the function to each item in the list. The (fun (x, _, _) -> x) unpacks the tuple and returns the first item.

HTH, Rob

If you wanted 2/3 of the values you could also do this:

let newList = oldList |> List.collect ( fun (a,b,_) -> [a;b])

The builtin function fst extracts the first element of a tuple, so you can just map fst over the list:

lst |> List.map fst

(* 'a*'b*'c list -> 'a list *)